Field of the Invention
The present invention relates to an LN optical control device including an LN optical waveguide element in which an optical waveguide and a control electrode configured to control a light wave that propagates through the optical waveguide are formed in a substrate that is formed by using lithium niobate (LN), and a housing that accommodates the LN optical waveguide element in an air-tight sealing manner, and particularly to, an LN optical control device (referred to as a thin-plate LN optical control device) that uses the LN optical waveguide element that is thinned.
Description of Related Art
In an optical communication field or an optical measurement field, there is known an optical control device that uses LN having an electro-optic effect capable of realizing high-speed response. As one of LN optical control devices, there is known an LN optical modulator in which an LN optical waveguide element is accommodated in a housing. In the LN optical waveguide element, a Mach-Zehnder type optical waveguide formed by thermal diffusion of titanium (Ti) and a control electrode configured to control a light wave that propagates through the optical waveguide are formed in an LN substrate.
An optical bandwidth as an example of the performance of the LN optical modulator has a trade-off relationship with a drive voltage. As an example of a method of realizing an LN optical modulator having low-drive-voltage performance in a relatively broadbandwidth, there is known a method of thinning an LN substrate in which the optical waveguide is formed or the LN optical waveguide element (refer to Japanese Laid-open Patent Publication No. 2010-85738).
The LN optical waveguide element is fixed to an inner side of the housing by using an adhesive. In addition, an electrical interconnection including the control electrode of the LN optical waveguide element is formed by using a metal. When being left as is in a moisture-containing atmosphere, an adhesive force of an organic material such as the adhesive may deteriorate. In addition, in a case where application of a voltage is performed continuously in the moisture-containing atmosphere, disconnection or short-circuiting due to migration may occur in the electrical interconnection.
In addition, DC drift is known as a phenomenon peculiar to the LN optical modulator. The DC drift is a phenomenon in which a voltage (referred to as “control voltage”) at an operation point for driving the LN optical modulator varies with the elapse of DC voltage application time when a DC voltage is continuously applied to a control electrode of the LN optical modulator (refer to Japanese Laid-open Patent Publication No. 7-152007).
To solve a problem related to reliability of the LN optical modulator, the following structure is used. In the structure, an atmosphere inside a housing, in which an LN optical waveguide element is accommodated, is substituted with an inert gas (for example, nitrogen, helium) that does not contain moisture, and the housing is air-tightly sealed to block an atmosphere on an outer side of the housing (refer to Hiroshi NAGATA and Naoki MITSUGI, “Mechanical Reliability of LiNbO3 Optical Modulators Hermetically Sealed in Stainless Steel Packages”, OPTICAL FIBER TECHNOLOGY, Volume 2, pages 216 to 224 (1996)).
The present inventors have prepared a thin-plate LN optical modulator in which an LN optical waveguide element (thin-plate LN optical waveguide element) that is thinned is accommodated in a housing having an air-tightly sealed structure in which an atmosphere inside the housing is substituted with an inert gas that does not contain moisture. DC drift of the thin-plate LN optical modulator has been evaluated. From the evaluation, when comparing the DC drift with DC drift of an LN optical modulator that is not thinned, it was confirmed that a DC drift amount (a fluctuation amount of a control voltage after a constant time at predetermined temperature and initial control voltage) increases, a variation in a DC drift amount between samples increases, and reproducibility (reproducibility of the amount and behavior of the DC drift when measuring the DC drift a plurality of times by using the same sample) is low.
In addition, with regard to an LN substrate, a crystal-grown boule is sliced in a thickness of approximately several hundreds of μm to 1 mm, and a surface of the LN substrate is polished to be flat. In a case where the thickness of the LN substrate is several hundred μm, a processing damage formed on the surface of the LN substrate can be removed or recovered by chemicals or by thermal annealing.
However, there is a concern that an LN substrate that is thinned to several tens of μm or less may be broken, and thus the LN substrate is fixed to a reinforcing substrate by using an adhesive. Accordingly, it is not easy to perform the above-described chemical and thermal treatments. In addition, mass productivity is low under mechanical processing conditions in which the processing damage does not occur, and thus an inspection process is necessary.